Directional display apparatus

ABSTRACT

A switchable privacy display comprises a spatial light modulator (SLM), a first switchable liquid crystal (LC) retarder and first passive retarder arranged between a first pair of polarisers and a second switchable LC retarder and second passive retarder arranged between a second pair of polarisers. Each switchable LC retarder comprises a homeotropic alignment layer and a homogeneous alignment layer. In privacy mode, on-axis light from the SLM is directed without loss, whereas off-axis light has reduced luminance to reduce the visibility of the display to off-axis snoopers. The display may achieve privacy operation in landscape and portrait orientations. Further, display reflectivity may be reduced for on-axis reflections of ambient light, while reflectivity may be increased for off-axis light to achieve increased visual security. In public mode, the liquid crystal retardance is adjusted so that off-axis luminance and reflectivity are unmodified. The display may be switched between day-time and night-time operation.

TECHNICAL FIELD

This disclosure generally relates to illumination from light modulation devices, and more specifically relates to optical stacks for providing control of illumination for use in display including privacy display and night-time display.

BACKGROUND

Privacy displays provide image visibility to a primary user that is typically in an on-axis position and reduced visibility of image content to a snooper, that is typically in an off-axis position. A privacy function may be provided by micro-louvre optical films that transmit some light from a display in an on-axis direction with low luminance in off-axis positions. However such films have high losses for head-on illumination and the micro-louvres may cause Moiré artefacts due to beating with the pixels of the spatial light modulator. The pitch of the micro-louvre may need selection for panel resolution, increasing inventory and cost.

Switchable privacy displays may be provided by control of the off-axis optical output.

Control may be provided by means of luminance reduction, for example by means of switchable backlights for a liquid crystal display (LCD) spatial light modulator. Display backlights in general employ waveguides and edge emitting sources. Certain imaging directional backlights have the additional capability of directing the illumination through a display panel into viewing windows. An imaging system may be formed between multiple sources and the respective window images. One example of an imaging directional backlight is an optical valve that may employ a folded optical system and hence may also be an example of a folded imaging directional backlight. Light may propagate substantially without loss in one direction through the optical valve while counter-propagating light may be extracted by reflection off tilted facets as described in U.S. Pat. No. 9,519,153, which is herein incorporated by reference in its entirety.

BRIEF SUMMARY

According to a first aspect of the present disclosure there is provided a display device comprising: a spatial light modulator; a display polariser arranged on a side of the spatial light modulator, the display polariser being a linear polariser; a first additional polariser arranged on the same side of the spatial light modulator as the display polariser, the first additional polariser being a linear polariser; at least one first polar control retarder arranged between the first additional polariser and the display polariser, a second additional polariser, the second additional polariser being a linear polariser; and at least one second polar control retarder, wherein either: the second additional polariser is arranged on the same side of the spatial light modulator as the first additional polariser outside the first additional polariser, and the at least one second polar control retarder is arranged between the first additional polariser and the second additional polariser; or the display device further comprises a backlight arranged to output light, the spatial light modulator comprises a transmissive spatial light modulator arranged to receive output light from the backlight, said display polariser is an input display polariser arranged on the input side of the spatial light modulator, and the display device further comprises an output display polariser arranged on the output side of the spatial light modulator, the second additional polariser is arranged on the output side of the spatial light modulator, and the at least one second polar control retarder is arranged between the second additional polariser and the output display polariser, wherein each of the at least one first polar control retarder and the at least one second polar control retarder comprises a respective switchable liquid crystal retarder comprising a layer of liquid crystal material and two surface alignment layers disposed adjacent to the layer of liquid crystal material and on opposite sides thereof, one of the surface alignment layers being arranged to provide homogenous alignment in the adjacent liquid crystal material and the other of the surface alignment layers being arranged to provide homeotropic alignment in the adjacent liquid crystal material. Advantageously a switchable privacy display may be provided with extended polar regions over which desirable security level may be achieved.

Said one of the surface alignment layers of the at least one first polar control retarder that is arranged to provide homogenous alignment in the adjacent liquid crystal material may have a pretilt having a pretilt direction with a component in the plane of the layer of liquid crystal material in a first direction. Said one of the surface alignment layers of the at least one second polar control retarder that is arranged to provide homogenous alignment in the adjacent liquid crystal material may have a pretilt having a pretilt direction with a component in the plane of the layer of liquid crystal material in a second direction. Said first and second directions may be crossed. Said first and second directions may be perpendicular. Advantageously polar regions with desirable security level may be achieved in both lateral and elevation directions. A switchable privacy display may be provided in landscape and portrait directions. In an automotive use, reflections from windscreens may be reduced.

Said one of the surface alignment layers of the at least one first polar control retarder that is arranged to provide homogenous alignment in the adjacent liquid crystal material may have a pretilt having a pretilt direction with a component in the plane of the layer of liquid crystal material in a first direction. Said one of the surface alignment layers of the at least one second polar control retarder that is arranged to provide homogenous alignment in the adjacent liquid crystal material may have a pretilt having a pretilt direction with a component in the plane of the layer of liquid crystal material in a second direction. Said first and second directions may be anti-parallel. Advantageously a switchable privacy display may be provided with high off-axis luminance in a public mode of operation and with high security level in a privacy mode of operation. Symmetric performance may be achieved in the elevation direction while privacy may be provided in the lateral direction for off-axis polar angles.

The display device may further comprise a reflective polariser, the reflective polariser being a linear polariser, and either: said display polariser may be an output display polariser arranged on the output side of the spatial light modulator, the second additional polariser is arranged on the same side of the spatial light modulator as the first additional polariser outside the first additional polariser, the at least one second polar control retarder is arranged between the first additional polariser and the second additional polariser, and the reflective polariser is arranged between the first additional polariser and the at least one second polar control retarder; or the display device further comprises a backlight arranged to output light, the spatial light modulator comprises a transmissive spatial light modulator arranged to receive output light from the backlight, said display polariser is an input display polariser arranged on the input side of the spatial light modulator, and the display device further comprises an output display polariser arranged on the output side of the spatial light modulator, the second additional polariser is arranged on the output side of the spatial light modulator, and the at least one second polar control retarder is arranged between the second additional polariser and the output display polariser, and the reflective polariser is arranged between the output display polariser and at least one second polar control retarder. Advantageously in a privacy mode of operation, increased display reflectivity may be provided. In ambient illuminance, increased security level of the display may be achieved for snooper locations.

The switchable liquid crystal retarder of the at least one first polar control retarder may have a retardance for light of a wavelength of 550 nm having a first retardance value and the switchable liquid crystal retarder of the at least one second polar control retarder has a retardance for light of a wavelength of 550 nm having a second retardance value, the first retardance value and the second retardance value being the same. Advantageously cost and complexity may be reduced.

The switchable liquid crystal retarder of the at least one first polar control retarder and the switchable liquid crystal retarder of the at least one second polar control retarder may each have a retardance for light of a wavelength of 550 nm in a range from 700 nm to 2500 nm. Advantageously luminance may be reduced over desirable polar regions; in embodiments comprising a reflective polariser reflectance may be increased over a wide polar area and security level may be provided at desirable levels over a wide polar region.

Each of the at least one first polar control retarder and at least one second polar control retarder may further comprise at least one passive compensation retarder. Advantageously the area of luminance reduction may be increased. Further the uniformity of luminance in public mode may be increased.

Each of the at least one passive compensation retarder of the at least one first polar control retarder and the at least one passive compensation retarder of the at least one second polar control retarder may be arranged on the same side of the switchable liquid crystal retarder as the surface alignment layers that is arranged to provide homeotropic alignment in the adjacent liquid crystal material. The at least one passive compensation retarder of the at least one first polar control retarder and the at least one passive compensation retarder of the at least one second polar control retarder may each comprise a passive uniaxial retarder having its optical axis perpendicular to the plane of the retarder. The passive uniaxial retarder may have a retardance for light of a wavelength of 550 nm in a range from −400 nm to −2100 nm. Advantageously the size of the polar region for desirable security level is increased.

The spatial light modulator may comprise an emissive spatial light modulator arranged to output light, the display polariser is an output display polariser arranged on the output side of the emissive spatial light modulator, the second additional polariser is arranged on the output side of the spatial light modulator outside the first additional polariser, and the at least one second polar control retarder is arranged between the first additional polariser and the second additional polariser. Advantageously an emissive switchable privacy display may be provided.

The emissive spatial light modulator may comprise an array of pixels arranged in a pixel layer, and the display device may further comprise a parallax barrier forming an array of apertures, wherein the parallax barrier may be separated from the pixel layer by a parallax distance along an axis along a normal to the plane of the pixel layer, each pixel being aligned with an aperture. The emissive spatial light modulator and parallax barrier has an output luminance profile having a full width half maximum that is at most 40 degrees. Advantageously a switchable privacy display may be provided with increased security level for off-axis snooper locations. An emissive display may be provided to operate in a privacy mode for both landscape and portrait operation.

The display device may comprise a backlight arranged to output light, and the spatial light modulator may comprise a transmissive spatial light modulator arranged to receive output light from the backlight. The backlight has an output luminance profile having a full width half maximum that is at most 40 degrees. Advantageously a switchable privacy display may be provided with increased security level for off-axis snooper locations. A transmissive display may be provided to operate in a privacy mode for both landscape and portrait operation. The polar area for desirable security level may be increased.

The display device may further comprise a backlight arranged to output light, and the spatial light modulator comprises a transmissive spatial light modulator arranged to receive output light from the backlight. Advantageously a privacy display may be provided for a transmissive displays. The backlight may be provided with reduced cone angle.

Said display polariser may be an input display polariser arranged on the input side of the spatial light modulator; the first additional polariser may be arranged between the backlight and the input display polariser; and the second additional polariser may be arranged on the same side of the spatial light modulator as the first additional polariser between the backlight and the first additional polariser, and the at least one second polar control retarder may be arranged between the first additional polariser and the second additional polariser. Advantageously the visibility of frontal reflections from the front surface of the display device may be reduced.

Any of the aspects of the present disclosure may be applied in any combination.

Embodiments of the present disclosure may be used in a variety of optical systems. The embodiments may include or work with a variety of projectors, projection systems, optical components, displays, microdisplays, computer systems, processors, self-contained projector systems, visual and/or audio-visual systems and electrical and/or optical devices. Aspects of the present disclosure may be used with practically any apparatus related to optical and electrical devices, optical systems, presentation systems or any apparatus that may contain any type of optical system. Accordingly, embodiments of the present disclosure may be employed in optical systems, devices used in visual and/or optical presentations, visual peripherals and so on and in a number of computing environments.

Before proceeding to the disclosed embodiments in detail, it should be understood that the disclosure is not limited in its application or creation to the details of the particular arrangements shown, because the disclosure is capable of other embodiments. Moreover, aspects of the disclosure may be set forth in different combinations and arrangements to define embodiments unique in their own right. Also, the terminology used herein is for the purpose of description and not of limitation.

These and other advantages and features of the present disclosure will become apparent to those of ordinary skill in the art upon reading this disclosure in its entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example in the accompanying FIGURES, in which like reference numbers indicate similar parts, and in which:

FIG. 1A is a schematic diagram illustrating in side perspective view a switchable privacy display for use in ambient illumination comprising an emissive spatial light modulator, a first polar control retarder arranged between the display polariser of the emissive spatial light modulator and a first additional polariser; and a reflective polariser and second polar control retarder arranged between the first additional polariser and a second additional polariser;

FIG. 1B is a schematic diagram illustrating in front perspective view an arrangement polarisers and polar control retarders for the embodiment of FIG. 1A;

FIG. 1C is a schematic diagram illustrating in side perspective view the switchable privacy display of FIG. 1A wherein the reflective polariser is omitted and the first additional polariser is a reflective polariser;

FIG. 2 is a schematic diagram illustrating in side perspective view of an alternative structure of spatial light modulator for use in the arrangement of FIG. 1A further comprising a patterned barrier mask:

FIG. 3 is a schematic diagram illustrating in side perspective view of an alternative structure of spatial light modulator for use in the arrangement of FIG. 1A comprising a transmissive spatial light modulator and a backlight:

FIG. 4A is a schematic diagram illustrating in side perspective view a switchable privacy display for use in ambient illumination comprising the transmissive spatial light modulator and backlight of FIG. 3; a reflective polariser and a first polar control retarder arranged between the output display polariser of the spatial light modulator and a first additional polariser; and a second polar control retarder arranged between the input display polariser of the spatial light modulator and a second additional polariser;

FIG. 4B is a schematic diagram illustrating in front perspective view an arrangement polarisers and polar control retarders for the embodiment of FIG. 4A;

FIG. 5A is a schematic diagram illustrating in side perspective view a switchable privacy display comprising the transmissive spatial light modulator and backlight of FIG. 3; a first polar control retarder arranged between the input display polariser of the spatial light modulator and a first additional polariser; and a second polar control retarder arranged between the first additional polariser and a second additional polariser:

FIG. 5B is a schematic diagram illustrating in front perspective view an arrangement polarisers and polar control retarders for the embodiment of FIG. 1A;

FIG. 6A is a schematic diagram illustrating in side perspective view a structure of a polar control retarder wherein the polar control retarder comprises a passive C plate and an active liquid crystal layer comprising a homeotropic alignment layer and a homogeneous alignment layer, wherein the pretilt directions of the alignment layers have a component in the plane of the alignment layers that are antiparallel, and the components are oriented in a first direction in the plane of the alignment layers;

FIG. 6B is a schematic diagram illustrating in side perspective view a structure of a polar control retarder that is anti-parallel to the arrangement of FIG. 6A;

FIG. 6C is a schematic diagram illustrating in side perspective view a structure of a polar control retarder wherein the polar control retarder comprises a passive C plate and an active liquid crystal layer comprising a homeotropic alignment layer and a homogeneous alignment layer, wherein the pretilt directions of the alignment layers have a component in the plane of the alignment layers that are antiparallel, and the components are oriented in a second direction in the plane of the alignment layers that is orthogonal to the orientation of the components in FIG. 6A;

FIG. 7A is a schematic diagram illustrating in side view the operation of the arrangement of FIG. 1A for transmitted light from the spatial light modulator in the public mode of operation;

FIG. 7B is a schematic diagram illustrating in side view the operation of the arrangement of FIG. 1A for ambient light in the public mode of operation;

FIG. 7C is a schematic diagram illustrating in side view the operation of the arrangement of FIG. 1A for transmitted light from the spatial light modulator in a privacy mode of operation with high reflectivity of ambient light;

FIG. 7D is a schematic diagram illustrating in side view the operation of the arrangement of FIG. 1A for ambient light in a privacy mode of operation with high reflectivity of ambient light;

FIG. 8A is a schematic diagram illustrating in front perspective view an arrangement polarisers and polar control retarders for the embodiment of FIG. 1A wherein the first and second polar control retarders are anti-parallel;

FIG. 8B is a graph illustrating a simulated polar profile of luminance output of an emissive spatial light modulator:

FIG. 8C is a graph illustrating a simulated polar profile of transmission of the second polar control retarder of FIG. 8A arranged between the first and second additional polarisers wherein the electric vector transmission directions of the polarisers are parallel;

FIG. 8D is a graph illustrating a simulated polar profile of reflectivity of the second polar control retarder of FIG. 8A arranged between a reflective polariser and the second additional polariser wherein the electric vector transmission directions of the polarisers are parallel;

FIG. 8E is a graph illustrating a simulated polar profile of the total reflectivity comprising the reflectivity of FIG. 8D and the Fresnel reflectivity from the front surface of the display device;

FIG. 8F is a graph illustrating a simulated polar profile of transmission of the first polar control retarder of FIG. 8A arranged between the display polariser and the first additional polariser wherein the electric vector transmission directions of the polarisers are parallel;

FIG. 8G is a graph illustrating a simulated polar profile of the logarithm of total output luminance of the spatial light modulator and first and second polar control retarders of FIG. 8A;

FIG. 8H is a graph illustrating a simulated polar profile of the security level, S of the arrangement of FIG. 8A for an ambient illuminance measured in lux that is twice the head-on display luminance measured in nits;

FIG. 8I is a graph illustrating a simulated lateral profile for zero degrees elevation of the visual security factor of the arrangement of FIG. 8A for an ambient illuminance measured in lux that is twice the head-on display luminance measured in nits:

FIG. 8J is a graph illustrating a simulated polar profile of the security level, S of the arrangement of FIG. 8A for an ambient illuminance measured in lux that is twice the head-on display luminance measured in nits and operated in public mode;

FIG. 9A is a schematic diagram illustrating in front perspective view an arrangement polarisers and polar control retarders for the embodiment of FIG. 4A wherein the first and second polar control retarders are anti-parallel;

FIG. 9B is a graph illustrating a simulated polar profile of luminance output of a transmissive spatial light modulator that is illuminated by a collimated backlight:

FIG. 9C is a graph illustrating a simulated polar profile of transmission of the first polar control retarder of FIG. 9A arranged between the output display polariser and the first additional polariser wherein the electric vector transmission directions of the polarisers are parallel;

FIG. 9D is a graph illustrating a simulated polar profile of reflectivity of the first polar control retarder of FIG. 9A arranged between the reflective polariser and the first additional polariser wherein the electric vector transmission directions of the polarisers are parallel;

FIG. 9E is a graph illustrating a simulated polar profile of the total reflectivity comprising the reflectivity of FIG. 9D and the Fresnel reflectivity from the front surface of the display device:

FIG. 9F is a graph illustrating a simulated polar profile of transmission of the second polar control retarder of FIG. 9A arranged between the input display polariser and the second additional polariser wherein the electric vector transmission directions of the polarisers are parallel:

FIG. 9G is a graph illustrating a simulated polar profile of the logarithm of total output luminance of the spatial light modulator and first and second polar control retarders of FIG. 9A;

FIG. 9H is a graph illustrating a simulated polar profile of the security level, S of the arrangement of FIG. 9A for an ambient illuminance measured in lux that is twice the head-on display luminance measured in nits:

FIG. 9I is a graph illustrating a simulated lateral profile for zero degrees elevation of the visual security factor of the arrangement of FIG. 9A for an ambient illuminance measured in lux that is twice the head-on display luminance measured in nits:

FIG. 9J is a graph illustrating a simulated polar profile of the security level, S of the arrangement of FIG. 9A for an ambient illuminance measured in lux that is twice the head-on display luminance measured in nits and operated in public mode;

FIG. 10A is a schematic diagram illustrating in front perspective view an arrangement polarisers and polar control retarders for the embodiment of FIG. 5A wherein the first and second polar control retarders are anti-parallel:

FIG. 10B is a graph illustrating a simulated polar profile of luminance output of a transmissive spatial light modulator that is illuminated by a collimated backlight:

FIG. 10C is a graph illustrating a simulated polar profile of transmission of the first polar control retarder of FIG. 10A arranged between the input display polariser and the first additional polariser wherein the electric vector transmission directions of the polarisers are parallel;

FIG. 10D is a graph illustrating a simulated polar profile of reflectivity of the first polar control retarder of FIG. 10A arranged between the input display polariser and the first additional polariser:

FIG. 10E is a graph illustrating a simulated polar profile of the total reflectivity comprising the reflectivity of FIG. 10D and the Fresnel reflectivity from the front surface of the display device;

FIG. 10F is a graph illustrating a simulated polar profile of transmission of the second polar control retarder of FIG. 10A arranged between the first and second additional polarisers;

FIG. 10G is a graph illustrating a simulated polar profile of the logarithm of total output luminance of the spatial light modulator and first and second polar control retarders of FIG. 10A;

FIG. 10H is a graph illustrating a simulated polar profile of the security level, S of the arrangement of FIG. 10A for an ambient illuminance measured in lux that is twice the head-on display luminance measured in nits:

FIG. 10I is a graph illustrating a simulated lateral profile for zero degrees elevation of the visual security factor of the arrangement of FIG. 10A for an ambient illuminance measured in lux that is twice the head-on display luminance measured in nits;

FIG. 11A is a schematic diagram illustrating in front perspective view an arrangement polarisers and polar control retarders for the embodiment of FIG. 4A wherein the first and second polar control retarders are crossed;

FIG. 11B is a graph illustrating a simulated polar profile of luminance output of a transmissive spatial light modulator that is illuminated by a collimated backlight:

FIG. 11C is a graph illustrating a simulated polar profile of transmission of the first polar control retarder of FIG. 1I A arranged between the output display polariser and the first additional polariser wherein the electric vector transmission directions of the polarisers are parallel;

FIG. 11D is a graph illustrating a simulated polar profile of reflectivity of the first polar control retarder of FIG. 11A arranged between the reflective polariser and the first additional polariser wherein the electric vector transmission directions of the polarisers are parallel;

FIG. 11E is a graph illustrating a simulated polar profile of the total reflectivity comprising the reflectivity of FIG. 11D and the Fresnel reflectivity from the front surface of the display device;

FIG. 11F is a graph illustrating a simulated polar profile of transmission of the second polar control retarder of FIG. 11A arranged between the input display polariser and the second additional polariser wherein the electric vector transmission directions of the polarisers are parallel;

FIG. 11G is a graph illustrating a simulated polar profile of the logarithm of total output luminance of the spatial light modulator and first and second polar control retarders of FIG. 11A;

FIG. 11H is a graph illustrating a simulated polar profile of the security level. S of the arrangement of FIG. 11A for an ambient illuminance measured in lux that is twice the head-on display luminance measured in nits and the display is operated in landscape orientation;

FIG. 11I is a graph illustrating a simulated polar profile of the security level, S of the arrangement of FIG. 11A for an ambient illuminance measured in lux that is twice the head-on display luminance measured in nits and the display is operated in portrait orientation;

FIG. 12A is a schematic diagram illustrating in front perspective view an arrangement polarisers and polar control retarders for the embodiment of FIG. 1A wherein the first and second polar control retarders are crossed and the spatial light modulator is provided by the alternative of FIG. 2;

FIG. 12B is a graph illustrating a simulated polar profile of luminance output of an emissive spatial light modulator without the barrier structure of FIG. 2;

FIG. 12C is a graph illustrating a simulated polar profile of transmission of the barrier structure of FIG. 2 of light from the pixels of the emissive spatial light modulator:

FIG. 12D is a graph illustrating a simulated polar profile of transmission of the second polar control retarder of FIG. 12A arranged between the first and second additional polarisers wherein the electric vector transmission directions of the polarisers are parallel;

FIG. 12E is a graph illustrating a simulated polar profile of reflectivity of the second polar control retarder of FIG. 12A arranged between a reflective polariser and the second additional polariser wherein the electric vector transmission directions of the polarisers are parallel;

FIG. 12F is a graph illustrating a simulated polar profile of the total reflectivity comprising the reflectivity of FIG. 12E and the Fresnel reflectivity from the front surface of the display device;

FIG. 12G is a graph illustrating a simulated polar profile of transmission of the first polar control retarder of FIG. 12A arranged between the display polariser and the first additional polariser wherein the electric vector transmission directions of the polarisers are parallel;

FIG. 12H is a graph illustrating a simulated polar profile of the logarithm of total output luminance of the spatial light modulator and first and second polar control retarders of FIG. 12A;

FIG. 12I is a graph illustrating a simulated polar profile of the security level, S of the arrangement of FIG. 12A for an ambient illuminance measured in lux that is twice the head-on display luminance measured in nits:

FIG. 13 is a schematic diagram illustrating in side perspective view a switchable privacy display component for use with a spatial light modulator comprising a first polar control retarder and a first additional polariser, a reflective polariser; and a second polar control retarder arranged between the first additional polariser and a second additional polariser;

FIG. 14 is a key for the alternative stacking arrangements of FIGS. 15A-F, FIGS. 16A-F. FIGS. 17A-C and FIGS. 18A-F;

FIG. 15A, FIG. 15B, FIG. 15C, FIG. 15D, FIG. 15E, and FIG. 15F are schematic diagrams illustrating in side view alternatives for optical component stackings for a switchable privacy display wherein the first and second polar control retarders are arranged to receive light from a transmissive spatial light modulator and backlight;

FIG. 16A, FIG. 16B, FIG. 16C, FIG. 16D, FIG. 16E, and FIG. 16F are schematic diagrams illustrating in side view alternatives for optical component stackings for a switchable privacy display wherein one of the first and second polar control retarders is arranged to receive light from the spatial light modulator and the transmissive spatial light modulator is arranged to receive light from the other of the first and second polar control retarders and a backlight;

FIG. 17A, FIG. 17B, and FIG. 17C are schematic diagrams illustrating in side view alternatives for optical component stackings for a switchable privacy display wherein the transmissive spatial light modulator is arranged to receive light from the first and second polar control retarders and a backlight; and

FIG. 18A, FIG. 18B, FIG. 18C, FIG. 18D, FIG. 18E, and FIG. 18F are schematic diagrams illustrating in side view alternatives for optical component stackings for a switchable privacy display wherein the first and second polar control retarders are arranged to receive light from an emissive spatial light modulator.

DETAILED DESCRIPTION

Terms related to optical retarders for the purposes of the present disclosure will now be described.

In a layer comprising a uniaxial birefringent material there is a direction governing the optical anisotropy whereas all directions perpendicular to it (or at a given angle to it) have equivalent birefringence.

The optical axis of an optical retarder refers to the direction of propagation of a light ray in the uniaxial birefringent material in which no birefringence is experienced. This is different from the optical axis of an optical system which may for example be parallel to a line of symmetry or normal to a display surface along which a principal ray propagates.

For light propagating in a direction orthogonal to the optical axis, the optical axis is the slow axis when linearly polarized light with an electric vector direction parallel to the slow axis travels at the slowest speed. The slow axis direction is the direction with the highest refractive index at the design wavelength. Similarly the fast axis direction is the direction with the lowest refractive index at the design wavelength.

For positive dielectric anisotropy uniaxial birefringent materials the slow axis direction is the extraordinary axis of the birefringent material. For negative dielectric anisotropy uniaxial birefringent materials the fast axis direction is the extraordinary axis of the birefringent material.

The terms half a wavelength and quarter a wavelength refer to the operation of a retarder for a design wavelength λ₀ that may typically be between 500 nm and 570 nm. In the present illustrative embodiments exemplary retardance values are provided for a wavelength of 550 nm unless otherwise specified.

The retarder provides a phase shift between two perpendicular polarization components of the light wave incident thereon and is characterized by the amount of relative phase, Γ, that it imparts on the two polarization components; which is related to the birefringence Δn and the thickness d of the retarder by Γ=2·π·Δn·d/π ₀  eqn. 1

In eqn. 1, Δn is defined as the difference between the extraordinary and the ordinary index of refraction, i.e. Δn=n _(c) −n _(o)  eqn. 2

For a half-wave retarder, the relationship between d, Δn, and λ₀ is chosen so that the phase shift between polarization components is Γ=π. For a quarter-wave retarder, the relationship between d, Δn, and λ₀ is chosen so that the phase shift between polarization components is Γ=π/2.

The term half-wave retarder herein typically refers to light propagating normal to the retarder and normal to the spatial light modulator.

Some aspects of the propagation of light rays through a transparent retarder between a pair of polarisers will now be described.

The state of polarisation (SOP) of a light ray is described by the relative amplitude and phase shift between any two orthogonal polarization components. Transparent retarders do not alter the relative amplitudes of these orthogonal polarisation components but act only on their relative phase. Providing a net phase shift between the orthogonal polarisation components alters the SOP whereas maintaining net relative phase preserves the SOP. In the current description, the SOP may be termed the polarisation state.

A linear SOP has a polarisation component with a non-zero amplitude and an orthogonal polarisation component which has zero amplitude.

A linear polariser transmits a unique linear SOP that has a linear polarisation component parallel to the electric vector transmission direction of the linear polariser and attenuates light with a different SOP. The term “electric vector transmission direction” refers to a non-directional axis of the polariser parallel to which the electric vector of incident light is transmitted, even though the transmitted “electric vector” always has an instantaneous direction. The term “direction” is commonly used to describe this axis.

Absorbing polarisers are polarisers that absorb one polarisation component of incident light and transmit a second orthogonal polarisation component. Examples of absorbing linear polarisers are dichroic polarisers.

Reflective polarisers are polarisers that reflect one polarisation component of incident light and transmit a second orthogonal polarisation component. Examples of reflective polarisers that are linear polarisers are multilayer polymeric film stacks such as DBEF™ or APF™ from 3M Corporation, or wire grid polarisers such as ProFlux™ from Moxtek. Reflective linear polarisers may further comprise cholesteric reflective materials and a quarter waveplate arranged in series.

A retarder arranged between a linear polariser and a parallel linear analysing polariser that introduces no relative net phase shift provides full transmission of the light other than residual absorption within the linear polariser.

A retarder that provides a relative net phase shift between orthogonal polarisation components changes the SOP and provides attenuation at the analysing polariser.

In the present disclosure an ‘A-plate’ refers to an optical retarder utilizing a layer of birefringent material with its optical axis parallel to the plane of the layer.

A ‘positive A-plate’ refers to positively birefringent A-plates, i.e. A-plates with a positive Δn.

In the present disclosure a ‘C-plate’ refers to an optical retarder utilizing a layer of birefringent material with its optical axis perpendicular to the plane of the layer. A ‘positive C-plate’ refers to positively birefringent C-plate, i.e. a C-plate with a positive Δn. A ‘negative C-plate’ refers to a negatively birefringent C-plate, i.e. a C-plate with a negative Δn.

‘O-plate’ refers to an optical retarder utilizing a layer of birefringent material with its optical axis having a component parallel to the plane of the layer and a component perpendicular to the plane of the layer. A ‘positive O-plate’ refers to positively birefringent O-plates, i.e. O-plates with a positive Δn.

Achromatic retarders may be provided wherein the material of the retarder is provided with a retardance Δn, d that varies with wavelength λ as Δn·d/λ=κ  eqn. 3

where κ is substantially a constant.

Examples of suitable materials include modified polycarbonates from Teijin Films. Achromatic retarders may be provided in the present embodiments to advantageously minimise color changes between polar angular viewing directions which have low luminance reduction and polar angular viewing directions which have increased luminance reductions as will be described below.

Various other terms used in the present disclosure related to retarders and to liquid crystals will now be described.

A liquid crystal cell has a retardance given by Δn, d where Δn is the birefringence of the liquid crystal material in the liquid crystal cell and d is the thickness of the liquid crystal cell, independent of the alignment of the liquid crystal material in the liquid crystal cell.

Homogeneous alignment refers to the alignment of liquid crystals in switchable liquid crystal displays where molecules align substantially parallel to a substrate. Homogeneous alignment is sometimes referred to as planar alignment. Homogeneous alignment may typically be provided with a small pre-tilt such as 2 degrees, so that the molecules at the surfaces of the alignment layers of the liquid crystal cell are slightly inclined as will be described below. Pretilt is arranged to minimise degeneracies in switching of cells.

In the present disclosure, homeotropic alignment is the state in which rod-like liquid crystalline molecules align substantially perpendicularly to the substrate. In discotic liquid crystals homeotropic alignment is defined as the state in which an axis of the column structure, which is formed by disc-like liquid crystalline molecules, aligns perpendicularly to a surface. In homeotropic alignment, pretilt is the tilt angle of the molecules that are close to the alignment layer and is typically close to 90 degrees and for example may be 88 degrees.

In a twisted liquid crystal layer a twisted configuration (also known as a helical structure or helix) of nematic liquid crystal molecules is provided. The twist may be achieved by means of a non-parallel alignment of alignment layers. Further, cholesteric dopants may be added to the liquid crystal material to break degeneracy of the twist direction (clockwise or anti-clockwise) and to further control the pitch of the twist in the relaxed (typically undriven) state. A supertwisted liquid crystal layer has a twist of greater than 180 degrees. A twisted nematic layer used in spatial light modulators typically has a twist of 90 degrees.

Liquid crystal molecules with positive dielectric anisotropy are switched from a homogeneous alignment (such as an A-plate retarder orientation) to a homeotropic alignment (such as a C-plate or O-plate retarder orientation) by means of an applied electric field.

Liquid crystal molecules with negative dielectric anisotropy are switched from a homeotropic alignment (such as a C-plate or O-plate retarder orientation) to a homogeneous alignment (such as an A-plate retarder orientation) by means of an applied electric field.

Rod-like molecules have a positive birefringence so that n_(e)>n_(o) as described in eqn. 2. Discotic molecules have negative birefringence so that n_(e)<n_(o).

Positive retarders such as A-plates, positive O-plates and positive C-plates may typically be provided by stretched films or rod-like liquid crystal molecules. Negative retarders such as negative C-plates may be provided by stretched films or discotic like liquid crystal molecules.

Parallel liquid crystal cell alignment refers to the alignment direction of homogeneous alignment layers being parallel or more typically antiparallel. In the case of pre-tilted homeotropic alignment, the alignment layers may have components that are substantially parallel or antiparallel. Hybrid aligned liquid crystal cells may have one homogeneous alignment layer and one homeotropic alignment layer. Twisted liquid crystal cells may be provided by alignment layers that do not have parallel alignment, for example oriented at 90 degrees to each other.

Transmissive spatial light modulators may further comprise retarders between the input display polariser and the output display polariser for example as disclosed in U.S. Pat. No. 8,237,876, which is herein incorporated by reference in its entirety. Such retarders (not shown) are in a different place to the passive retarders of the present embodiments. Such retarders compensate for contrast degradations for off-axis viewing locations, which is a different effect to the luminance reduction for off-axis viewing positions of the present embodiments.

A private mode of operation of a display is one in which an observer sees a low contrast sensitivity such that an image is not clearly visible. Contrast sensitivity is a measure of the ability to discern between luminances of different levels in a static image. Inverse contrast sensitivity may be used as a measure of visual security, in that a high visual security level (VSL) corresponds to low image visibility.

For a privacy display providing an image to an observer, visual security may be given as: V=(Y+R)/(Y−K)  eqn. 4

where V is the visual security level (VSL), Y is the luminance of the white state of the display at a snooper viewing angle, K is the luminance of the black state of the display at the snooper viewing angle and R is the luminance of reflected light from the display.

Panel contrast ratio is given as: C=Y/K  eqn. 5

so the visual security level may be further given as: V=(P·Y _(max) +I·ρ/π)/(P·(Y _(max) −Y _(max) /C))  eqn. 6 where: Y_(max) is the maximum luminance of the display; P is the off-axis relative luminance typically defined as the ratio of luminance at the snooper angle to the maximum luminance Y_(max); C is the image contrast ratio; ρ is the surface reflectivity; and I is the illuminance. The units of Y_(max) are the units of I divided by solid angle in units of steradian.

The luminance of a display varies with angle and so the maximum luminance of the display Y_(max) occurs at a particular angle that depends on the configuration of the display.

In many displays, the maximum luminance Y_(max) occurs head-on, i.e. normal to the display. Any display device disclosed herein may be arranged to have a maximum luminance Y_(max), that occurs head-on, in which case references to the maximum luminance of the display device Y_(max) may be replaced by references to the luminance normal to the display device.

Alternatively, any display described herein may be arranged to have a maximum luminance Y_(max) that occurs at a polar angle to the normal to the display device that is greater than 0°. By way of example, the maximum luminance Y_(max) may occur may at a non-zero polar angle and at an azimuth angle that has for example zero lateral angle so that the maximum luminance is for an on-axis user that is looking down on to the display device. The polar angle may for example be 10 degrees and the azimuthal angle may be the northerly direction (90 degrees anti-clockwise from easterly direction). The viewer may therefore desirably see a high luminance at typical non-normal viewing angles.

The off-axis relative luminance, P is sometimes referred to as the privacy level. However, such privacy level P describes relative luminance of a display at a given polar angle compared to head-on luminance, and in fact is not a measure of privacy appearance.

The illuminance, I is the luminous flux per unit area that is incident on the display and reflected from the display towards the observer location. For Lambertian illuminance, and for displays with a Lambertian front diffuser illuminance I is invariant with polar and azimuthal angles. For arrangements with a display with non-Lambertian front diffusion arranged in an environment with directional (non-Lambertian) ambient light, illuminance I varies with polar and azimuthal angle of observation.

Thus in a perfectly dark environment, a high contrast display has VSL of approximately 1.0. As ambient illuminance increases, the perceived image contrast degrades, VSL increases and a private image is perceived.

For typical liquid crystal displays the panel contrast C is above 100:1 for almost all viewing angles. allowing the visual security level to be approximated to: V=1+I·ρ/(π·P·Y _(max))  eqn. 7

In the present embodiments, in addition to the exemplary definition of eqn. 4, other measurements of visual security level, V may be provided, for example to include the effect on image visibility to a snooper of snooper location, image contrast, image colour and white point and subtended image feature size. Thus the visual security level may be a measure of the degree of privacy of the display but may not be restricted to the parameter V.

The perceptual image security may be determined from the logarithmic response of the eye, such that S=log₁₀(V)  eqn. 8 Desirable limits for S were determined in the following manner. In a first step a privacy display device was provided. Measurements of the variation of privacy level. P(θ) of the display device with polar viewing angle and variation of reflectivity ρ(θ) of the display device with polar viewing angle were made using photopic measurement equipment. A light source such as a substantially uniform luminance light box was arranged to provide illumination from an illuminated region that was arranged to illuminate the privacy display device along an incident direction for reflection to a viewer positions at a polar angle of greater than 0° to the normal to the display device. The variation I(θ) of illuminance of a substantially Lambertian emitting lightbox with polar viewing angle was determined by and measuring the variation of recorded reflective luminance with polar viewing angle taking into account the variation of reflectivity ρ(θ). The measurements of P(θ), r(θ) and I(θ) were used to determine the variation of Security Factor S(θ) with polar viewing angle along the zero elevation axis.

In a second step a series of high contrast images were provided on the privacy display including (i) small text images with maximum font height 3 mm, (ii) large text images with maximum font height 30 mm and (iii) moving images.

In a third step each observer (with eyesight correction for viewing at 1000 mm where appropriate) viewed each of the images from a distance of 1000 mm, and adjusted their polar angle of viewing at zero elevation until image invisibility was achieved for one eye from a position near on the display at or close to the centre-line of the display. The polar location of the observer's eye was recorded. From the relationship S(θ), the security factor at said polar location was determined. The measurement was repeated for the different images, for various display luminance Y_(max), different lightbox illuminance I(θ=0), for different background lighting conditions and for different observers.

From the above measurements S<1.0 provides low or no visual security, 1.0≤S<1.5 provides visual security that is dependent on the contrast, spatial frequency and temporal frequency of image content, 1.5≤S<1.8 provides acceptable image invisibility (that is no image contrast is observable) for most images and most observers and S≥1.8 provides full image invisibility, independent of image content for all observers.

In practical display devices, this means that it is desirable to provide a value of S for an off-axis viewer who is a snooper that meets the relationship S≥S_(min) where: S_(min) has a value of 1.0 or more to achieve the effect that the off-axis viewer cannot perceive the displayed image; S_(min) has a value of 1.5 or more to achieve the effect that the displayed image is invisible, i.e. the viewer cannot perceive even that an image is being displayed, for most images and most observers; or S_(min) has a value of 1.8 or more to achieve the effect that the displayed image is invisible independent of image content for all observers.

In comparison to privacy displays, desirably wide angle displays are easily observed in standard ambient illuminance conditions. One measure of image visibility is given by the contrast sensitivity such as the Michelson contrast which is given by: M=(I _(max) −I _(min))/(I _(max) +I _(min))  eqn. 9 and so: M=((Y+R)−(K+R))/((Y+R)+(K+R))=(Y−K)/(Y+K+2·R)  eqn. 10

Thus the visual security level (VSL), V is equivalent (but not identical to) 1/M. In the present discussion, for a given off-axis relative luminance, P the wide angle image visibility, W is approximated as W=1/V=1/(1+I·p/(π·P·Y _(max)))  eqn. 11

The above discussion focusses on reducing visibility of the displayed image to an off-axis viewer who is a snooper, but similar considerations apply to visibility of the displayed image to the intended user of the display device who is typically on-axis. In this case, decrease of the level of the visual security level (VSL) V corresponds to an increase in the visibility of the image to the viewer. During observation S<0.1 may provide acceptable visibility of the displayed image. In practical display devices, this means that it is desirable to provide a value of S for an on-axis viewer who is the intended user of the display device that meets the relationship S≤S_(max), where S_(max) has a value of 0.1.

In the present discussion the colour variation Δε of an output colour (u_(w)′+Δu′, v_(w)′+Δv′) from a desirable white point (u_(w)′, v_(w)′) may be determined by the CIELUV colour difference metric, assuming a typical display spectral illuminant and is given by: Δε=(Δu′ ² +Δv′ ²)^(1/2)  eqn. 12

The structure and operation of various directional display devices will now be described. In this description, common elements have common reference numerals. It is noted that the disclosure relating to any element applies to each device in which the same or corresponding element is provided. Accordingly, for brevity such disclosure is not repeated.

FIG. 1A is a schematic diagram illustrating in side perspective view a switchable privacy display device 100 for use in ambient illumination 604 comprising an emissive spatial light modulator 48, a first polar control retarder 300A arranged between the display polariser 218 of the emissive spatial light modulator 48 and a first additional polariser 318A; and a reflective polariser 302 and second polar control retarder 300B arranged between the first additional polariser 318A and a second additional polariser 318B; and FIG. 1B is a schematic diagram illustrating in front perspective view an arrangement polarisers and polar control retarders for the embodiment of FIG. 1A.

The display device 100 comprises a spatial light modulator 48; wherein the spatial light modulator 48 comprises an emissive spatial light modulator 48 arranged to output light, the display polariser 218 is an output display polariser arranged on the output side of the emissive spatial light modulator 48, the display polariser 218 being a linear polariser.

A quarter waveplate 202 is arranged between the display polariser 218 and the pixel plane 214 to reduce frontal reflections from the pixel plane 214. Substrates 212, 216 are arranged to provide support of the pixel plane 214.

A first additional polariser 318A is arranged on the same side of the spatial light modulator 48 as the display polariser 218, the first additional polariser 318 being a linear polariser. The first additional polariser 318A is an absorbing polariser such as an iodine polariser on stretched PVA.

At least one first polar control retarder 300A is arranged between the first additional polariser 318A and the display polariser 218.

The display device 100 further comprises a second additional polariser 318B, the second additional polariser being a linear polariser; and at least one second polar control retarder 300B. The second additional polariser 318B is arranged on the output side of the spatial light modulator 48 outside the first additional polariser 318A, and the at least one second polar control retarder 318B is arranged between the first additional polariser 318A and the second additional polariser 31B.

Said display polariser 218 is an output display polariser arranged on the output side of the spatial light modulator 48, and the display device further comprises a reflective polariser 302 arranged between the first additional polariser 318A and at least one second polar control retarder 300B, the reflective polariser being a linear polariser.

Each of the at least one first polar control retarder 300A and the at least one second polar control retarder 300B comprises a respective switchable liquid crystal retarder 301A, 301B comprising a layer of liquid crystal material 314A, 314B, arranged between transparent substrates 312A, 312B and 316A, 316B respectively.

Each of the at least one first polar control retarder 300A and at least one second polar control retarder 300B further comprises at least one passive compensation retarder 330A. 330B respectively.

In an alternative embodiment (not shown), reflective polariser 302 may be omitted.

FIG. 1C is a schematic diagram illustrating in side perspective view the switchable privacy display device 100 of FIG. 1A wherein the reflective polariser 302 is omitted and the first additional polariser 318A is a reflective polariser. Advantageously thickness and cost may be reduced. Features of the embodiment of FIG. 1A not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

The spatial light modulator 48 may take any suitable form. Some possible alternatives are as follows.

FIG. 2 is a schematic diagram illustrating in side perspective view of an alternative for the structure of emissive spatial light modulator 48 for use in the arrangement of FIG. 1A. The spatial light modulator 48 further comprising a patterned barrier mask 700 comprising light absorbing region 704 and apertures 702 that are aligned with the pixels 220, 222, 224 of the pixel plane 214. The mask is separated by distance d from the pixel plane 214 and is aligned to the pixels so that the pixels have high luminance on axis and reduced luminance off-axis.

In operation the mask 700 is arranged to provide transmission of light ray 440 from pixel 224 in the normal direction to the spatial light modulator 48, and the aligned aperture 702 is arranged with an aperture size to provide high transmission. By comparison light rays 442 that are inclined at a non-zero polar angle, may be absorbed in the absorbing region 704. The separation d is provided to achieve a minimum transmission at a desirable polar angle in at least one azimuthal direction. Advantageously off-axis luminance is reduced, achieving increased security factor.

Further, reflectivity of the pixel plane may be reduced as incident ambient light is absorbed at the absorbing region 704. Quarter waveplate 202 of FIGS. 1A-1B may be omitted achieving reduced cost and complexity.

Features of the embodiment of FIG. 2 not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

Another alternative for the emissive spatial light modulator 48 for use in the arrangement of FIGS. 1A-B will now be described.

FIG. 3 is a schematic diagram illustrating in side perspective view of an alternative structure of spatial light modulator for use in the arrangement of FIG. 1A comprising a transmissive spatial light modulator 48 and a backlight 20 arranged to output light. The spatial light modulator 48 comprises a transmissive spatial light modulator arranged to receive output light from the backlight 20. Features of the embodiment of FIG. 1A not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

The backlight may comprise a light guide plate (LGP) 1, light extraction layers 5 and rear reflector 3. The light extraction layers may comprise diffusers, light turning films or prism films. Light may be provided from an array of light sources such as LEDs 15 arranged at the edge of the LGP 1.

The output may be provide a wide angle luminance profile such as achieved using crossed BEF™ films from 3M corporation and may have a full width half maximum of greater than 50 degrees. The output may provide a narrow angle profile, such backlights may be termed collimated backlights and have a full width half maximum luminance of less than 50 degrees, for example 30 degrees. Examples of collimated backlights are illustrated in U.S. Pat. No. 10,935,714, which is herein incorporated by reference in its entirety. The backlight may comprise other types of structure including mini-LED arrays and known light distribution optics to achieve desirable uniformity. The backlight 20 may be further provided with a micro-louvre array arranged to reduce off-axis luminance output from the backlight 20. Advantageously security factor, S may be improved in comparison to wide angle backlights.

Alternative arrangements of polar control retarders and additional polarisers will now be described for display devices 100 comprising backlights 20.

FIG. 4A is a schematic diagram illustrating in side perspective view a switchable privacy display device 100 for use in ambient illumination 604 comprising the transmissive spatial light modulator 48 and backlight 20 of FIG. 3; and FIG. 4B is a schematic diagram illustrating in front perspective view an arrangement polarisers and polar control retarders for the embodiment of FIG. 4A.

The display device 100 further comprises an input display polariser 210 arranged on the input side of the spatial light modulator 48, and the display device 100 further comprises an output display polariser 218 arranged on the output side of the spatial light modulator 48, the first additional polariser 318A is arranged on the input side of the spatial light modulator 48 and the first polar control retarder 300A is arranged between the first additional polariser 318A and the input display polariser 210. The second additional polariser 318B is arranged on the output side of the spatial light modulator 48, and the at least one second polar control retarder 300B is arranged between the second additional polariser 318B and the output display polariser 218.

Second polar control retarder 300B is arranged between the input display polariser 210 of the spatial light modulator 48 and a second additional polariser 318B. Reflective polariser 302 is arranged between the output display polariser 218 and the second polar control retarder 318B. In an alternative embodiment (not shown), reflective polariser 302 may be omitted.

Advantageously the separation of the output of the second additional polariser 318B to the pixel plane 214 is reduced in comparison to the arrangements of FIGS. 1A-B. Contrast of the image seen may be increased due to the reduced number of layers. An air gap may be provided between the input polariser 210 and second polar retarder 300B, advantageously reducing assembly cost and complexity.

Features of the embodiment of FIG. 4A not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

The embodiments of FIGS. 1A-B and FIG. 4A may alternatively be provided with no reflective polariser 302. Advantageously frontal reflections are reduced in privacy mode in environments where increased reflection is considered undesirable. Another alternative arrangement with no reflective polariser will now be described.

FIG. 5A is a schematic diagram illustrating in side perspective view a switchable privacy display device 100 comprising the transmissive spatial light modulator 48 and backlight 20 of FIG. 3; and FIG. 5B is a schematic diagram illustrating in front perspective view an arrangement polarisers and polar control retarders for the embodiment of FIG. 5A.

A first polar control retarder 300A is arranged between the input display polariser 210 of the spatial light modulator 48 and a first additional polariser 318A; and a second polar control retarder 300B is arranged between the first additional polariser 318A and a second additional polariser 318B.

Reflective polariser 302 is omitted. In some environments such as certain automotive environments, reflective operation may be undesirable and front of display reflectivity may be reduced. Further cost may be reduced.

In comparison to the arrangements of FIG. 1 and FIG. 4A the output display polariser 218 is the display device 100 output polariser. Advantageously diffusers may be arranged on the polariser 218 to provide increased image haze with reduced image blurring. Air gaps may be provided between the spatial light modulator input polariser 210 and plural retarders 300A, 300B. Advantageously image contrast is not degraded and assembly cost and complexity is reduced.

Features of the embodiment of FIG. 5A not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

Arrangements of liquid crystal alignment for use in the retarders 300A, 300B of FIG. 1, FIG. 4A and FIG. 5A will now be described.

FIG. 6A is a schematic diagram illustrating in side perspective view a structure of a polar control retarder 300 wherein the polar control retarder 300 comprises a passive C plate retarder 330 and an active liquid crystal layer 314.

Electrodes 413, 415 are arranged to apply a voltage from driver 350 across the liquid crystal material 421 in the layer 314. In a first driven state the liquid crystal molecules are arranged to provide no phase modification to input polarisation state in a normal direction to the polar control retarder and modified phase to an input polarisation state in directions at an angle to the normal direction to the polar control retarder 300. Such a driven state may be provided for privacy mode operation.

In a second driven state the liquid crystal molecules are arranged to provide no phase modification to input polarisation state in a normal direction to the polar control retarder and modified phase to an input polarisation state in directions at an angle to the normal direction to the polar control retarder 300. Such a driven state may be provided for public (or share) mode operation.

Two surface alignment layers are disposed adjacent to the layer of liquid crystal material and on opposite sides thereof wherein one of the surface alignment layers being arranged to provide homogenous alignment in the adjacent liquid crystal material and the other of the surface alignment layers being arranged to provide homeotropic alignment in the adjacent liquid crystal material. The alignment layers thus comprise a homeotropic alignment layer 417A and a homogeneous alignment layer 417B.

The pretilt directions 419A, 419B of the alignment layers have a component in the plane of the alignment layers 417A, 417B that are antiparallel. The pretilt directions 419A, 419B refer to the alignment of the liquid crystal molecules 421 that are adjacent to said layers. The components 419Ay and 419By are the in-plane components and are anti-parallel to each other. Component 419Az at the homeotropic alignment layer 417A is much greater than component 419Ay while component 419Bz at the homogeneous alignment layer 417B is much smaller than component 41By. The pretilt angle is the angle between the directions 419A and 419Ay, and between directions 419B and 419By respectively.

The components 419Ay, 419By are oriented in a first direction in the plane of the alignment layers, that is parallel to the y-axis.

FIG. 6B is a schematic diagram illustrating in side perspective view a structure of a polar control retarder that is anti-parallel to the arrangement of FIG. 6A. That is when provided in the arrangements of FIG. 1A, FIG. 4 and FIG. 5, in some of the embodiments described hereinbelow, one of the first and second polar control retarders 300A, 300B may be of the arrangement illustrated in FIG. 6A and one of the first and second polar control retarders 300A, 300B may be of the arrangement illustrated in FIG. 6B. Thus directions 419Ay in FIG. 6A is anti-parallel to direction 419Ay in FIG. 6B and direction 419By in FIG. 6A is anti-parallel to direction 419By in FIG. 6B.

FIG. 6C is a schematic diagram illustrating in side perspective view a structure of a polar control retarder 300 arranged between a display polariser 218 and an additional polariser 318 wherein the polar control retarder 300 comprises a passive C plate 330 and an active liquid crystal layer 314 comprising a homeotropic alignment layer 417A and a homogeneous alignment layer 417B, wherein the pretilt directions 419A, 419B of the alignment layers have a component 419Ax, 419Bx in the plane of the alignment layers that are antiparallel. The components 419Ax, 419Bx are oriented in a second direction in the plane of the alignment layers that is orthogonal to the orientation of the components 419Ay, 419By in FIG. 6A. That is when provided in the arrangements of FIG. 1A, FIG. 4 and FIG. 5, in some of the embodiments described hereinbelow, one of the first and second polar control retarders 300A, 300B may be of the arrangement illustrated in FIG. 6A and one of the first and second polar control retarders 300A, 300B may be of the arrangement illustrated in FIG. 6C. Thus directions 419Ay in FIG. 6A is orthogonal to direction 419Ax in FIG. 6B and direction 419By in FIG. 6A is anti-parallel to direction 419Bx in FIG. 6B.

Features of the embodiment of FIGS. 6A-C not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

Operation of polar control retarders between parallel polarisers is described further in U.S. Pat. No. 10,126,575 and in U.S. Patent Publ. No. 2019-0086706 (Atty. Ref No. 412101), both of which are herein incorporated by reference in their entireties. The operation of the polar control retarders in a public mode of operation will now be described.

FIG. 7A is a schematic diagram illustrating in side view the operation of the arrangement of FIG. 1A for transmitted light from the spatial light modulator in the public mode of operation. In the embodiments that will be described below, light ray 400 that is normal to the display (or in a head-on direction) is transmitted by the display polariser 219 with a polarisation state 360 that is unmodified by the polar control retarders 300A, 300B and polarisers 318A, 302 and 318B. Such light is transmitted with high luminance.

In public mode, rays 402 with a non-zero polar angle to the normal direction are also transmitted with the same polarisation state 360 that is substantially not modified by the polar control retarders 300A, 300B and polarisers 318A, 302 and 318B. The polar profile of luminance from the spatial light modulator may be substantially unmodified. Advantageously the display may be visible from a wide range of polar viewing positions and viewable by multiple display users.

FIG. 7B is a schematic diagram illustrating in side view the operation of the arrangement of FIG. 1A for incident ambient light 604 in the public mode of operation. Light rays 404, 406 are incident on the display device 100 with substantially unpolarised state 370. The polariser 318B provides a polarisation state 360 that is incident on the first polar control retarder and is substantially unmodified for head-on ray 404 and off-axis ray 406. Thus the light rays are substantially not reflected by the display are absorbed in the spatial light modulator 48 and backlight 20 if present. The display reflectivity is maintained at a low level for a wide range of viewing directions and advantageously a high contrast image is seen by multiple display users.

The operation of the polar control retarders in a private mode of operation will now be described.

FIG. 7C is a schematic diagram illustrating in side view the operation of the arrangement of FIG. 1A for transmitted light from the spatial light modulator in a privacy mode of operation with high reflectivity of ambient light. Head-on light ray 400 has a polarisation state 360 that is substantially unmodified by polar control retarders 300A, 300B. By comparison, off-axis light ray 402 has an output from the first polar control retarder that has an imparted phase difference to provide in general an elliptical state 362A. On incidence with first additional polariser 318A the luminance of the ray 402 is reduced with output state 360. Said light ray 402 is transmitted through reflective polariser 302 with small loss and is incident on the second polar control retarder 300B at which further phase modulation is provided and an output polarisation state 362B is achieved. Such state 362B is at least in part absorbed by second additional polariser 318B. Ray 402 is thus transmitted at off-axis polar locations with reduced luminance compared to the ray 402 in FIG. 7A.

FIG. 7D is a schematic diagram illustrating in side view the operation of the arrangement of FIG. 1A for ambient light in a privacy mode of operation with high reflectivity of ambient light. Head-on incident ambient light ray 404 is transmitted with substantial reflection from the reflective polariser 302.

By comparison light ray 406 undergoes a phase modulation at the polar control retarder 300B such that state 364 illuminates the reflective polariser. The resolved polarisation state 366 that is orthogonal to the electric vector transmission direction 303 of the reflective polariser 302 is reflected and is passed through the polar retarder such that polarisation state 368 is incident on to the second additional polariser. The component of the state 368 that is parallel to the electric vector transmission direction of the polariser 318B is thus transmitted. To an off-axis observer, the display appears to have increased reflectivity. Said increased reflectivity advantageously achieves increased security factor, S as described above.

Features of the embodiments of FIGS. 7A-D not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

The operation of FIGS. 7A-D is common to the arrangements of FIGS. 1A-B and FIG. 4A.

The embodiments of FIGS. 1A-B and FIG. 4A may further be provided with no reflective polariser 302. In such non-reflective structures and in FIG. 5A, the operation of FIGS. 7A and 7C only apply, that is the reflectivity does not increase in embodiments in which the reflective polariser 302 is omitted. Advantageously displays may be provided with low reflectivity in privacy mode of operation. For example displays for use in bright ambient environments such as for daytime operation in automotive vehicles may not provide a reflectivity enhancement in privacy mode.

Arrangements of polar control retarders to achieve desirable polar control of security factor, S in privacy mode will now be described.

FIG. 8A is a schematic diagram illustrating in front perspective view an arrangement polarisers and polar control retarders for the embodiment of FIG. 1A wherein the first and second polar control retarders 300A, 300B are anti-parallel. Features of the embodiment of FIG. 8A not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

Referring to the arrangement of FIG. 6A one of the surface alignment layers 417B of the at least one first polar control retarder 300A is arranged to provide homogenous alignment in the adjacent liquid crystal material 421 has a pretilt having a pretilt direction 419BB with a component 419BBy in the plane of the layer of liquid crystal material in a first direction.

One of the surface alignment layers 417B of the at least one second polar control retarder 300B that is arranged to provide homogenous alignment in the adjacent liquid crystal material 421 has a pretilt having a pretilt direction 419AB with a component 419ABy in the plane of the layer of liquid crystal material 421 in a second direction. In the arrangement of FIG. 8A, first and second directions are anti-parallel. In other words the direction 419ABy is opposite the direction 419BBy. Further the direction 419AAy with an in-plane alignment direction of 0 degrees is opposite the direction 419Bay with an in-plane alignment direction of 180 degrees.

The illustrative embodiment of TABLE 1 will now be discussed.

TABLE 1 In-plane Additional Additional passive Alignment alignment LC layer 314 passive retarder retarder 330 Item Layer type Pretilt direction retardance (range) 330 type retardance (range) 300B 419BB Homogeneous  2°  0° 1250 nm 419BA Homeotropic 88° 180° (700 nm~2500 nm) 330B Negative C- −1000 nm plate (−400 nm to −2100 nm) 300A 419AB Homogeneous  2° 180° 1250 nm 419AA Homeotropic 88°  0° (700 nm~2500 nm) 330A Negative C- −1000 nm plate (−400 nm to −2100 nm

Each of the at least one passive compensation retarder 330A of the at least one first polar control retarder 300A and the at least one passive compensation retarder 330B of the at least one second polar control retarder 300B is arranged on the same side of the switchable liquid crystal retarder 314A,B as the surface alignment layers 417A that is arranged to provide homeotropic alignment in the adjacent liquid crystal material 421. Advantageously the polar area of the luminance reduction and reflectivity increase is increased.

The at least one passive compensation retarder of the at least one first polar control retarder and the at least one passive compensation retarder of the at least one second polar control retarder each comprises a passive uniaxial retarder having its optical axis perpendicular to the plane of the retarder. The passive uniaxial retarder has a retardance for light of a wavelength of 550 nm in a range from −400 nm to −2100 nm.

The switchable liquid crystal retarder of the at least one first polar control retarder has a retardance for light of a wavelength of 550 nm having a first retardance value and the switchable liquid crystal retarder of the at least one second polar control retarder has a retardance for light of a wavelength of 550 nm having a second retardance value, the first retardance value and the second retardance value being the same. In other embodiments (not illustrated) the first and second retardance values may be different. Advantageously reduced privacy switch-on angle may be achieved.

The switchable liquid crystal retarder of the at least one first polar control retarder and the switchable liquid crystal retarder of the at least one second polar control retarder each have a retardance for light of a wavelength of 550 nm in a range from 700 nm to 2500 nm.

FIG. 8B is a graph illustrating a simulated polar profile of luminance output of an emissive spatial light modulator. As illustrated in FIG. 1A, the spatial light modulator 48 comprises an emissive spatial light modulator arranged to output light, the display polariser is an output display polariser 218 arranged on the output side of the emissive spatial light modulator 48, the second additional polariser 318B is arranged on the output side of the spatial light modulator 48 outside the first additional polariser 318A, and the at least one second polar control retarder 300B is arranged between the first additional polariser 318A and the second additional polariser 318B. FIG. 8B illustrates that the emissive spatial light modulator 48 has an output luminance profile having a full width half maximum that is at least 40 degrees, preferably at least 50 degrees.

In an alternative arrangement such as illustrated in FIG. 3 the display device may further comprises a backlight 20 arranged to output light, and the spatial light modulator 48 comprises a transmissive spatial light modulator arranged to receive output light from the backlight 20. The backlight may have an output luminance profile having a full width half maximum that is at least 40 degrees, preferably at least 50 degrees.

FIG. 8C is a graph illustrating a simulated polar profile of transmission of the second polar control retarder of FIG. 8A and TABLE 1 arranged between the first and second additional polarisers wherein the electric vector transmission directions of the polarisers are parallel; FIG. 8D is a graph illustrating a simulated polar profile of reflectivity of the second polar control retarder of FIG. 8A arranged between a reflective polariser and the second additional polariser wherein the electric vector transmission directions of the polarisers are parallel; FIG. 8E is a graph illustrating a simulated polar profile of the total reflectivity comprising the reflectivity of FIG. 8D and the Fresnel reflectivity from the front surface of the display device; FIG. 8F is a graph illustrating a simulated polar profile of transmission of the first polar control retarder of FIG. 8A and TABLE 1 arranged between the display polariser and the first additional polariser wherein the electric vector transmission directions of the polarisers are parallel; and FIG. 8G is a graph illustrating a simulated polar profile of the logarithm of total output luminance of the spatial light modulator and first and second polar control retarders of FIG. 8A and TABLE 1.

As described above, the security performance of the display can be determined using the polar variation of visual security level, S with polar viewing angle.

FIG. 8H is a graph illustrating a simulated polar profile of the security level, S of the arrangement of FIG. 8A and TABLE 1 for an ambient illuminance measured in lux that is twice the head-on display luminance measured in nits; and FIG. 8I is a graph illustrating a simulated lateral profile for zero degrees elevation of the visual security factor of the arrangement of FIG. 8A and TABLE 1 for an ambient illuminance measured in lux that is twice the head-on display luminance measured in nits. Thus the arrangement of FIG. 8A and TABLE 1 using a typical luminance profile of an emissive display of FIG. 1A can achieve a privacy switch-on of approximately 38 degrees. Advantageously an observer at 45 degrees can see substantially no image information, independent of image content for such ambient lighting conditions. Further the display shows substantially symmetric privacy performance.

FIG. 8J is a graph illustrating a simulated polar profile of the security level, S of the arrangement of FIG. 8A for an ambient illuminance measured in lux that is twice the head-on display luminance measured in nits and operated in public mode. Advantageously image visibility (S<0.1) is maintained to wide viewing angles so that the display can be easily seen with high contrast from polar viewing angles that are greater than 45 degrees.

FIG. 9A is a schematic diagram illustrating in front perspective view an arrangement polarisers and polar control retarders for the embodiment of FIG. 4A wherein the first and second polar control retarders are anti-parallel. Features of the embodiment of FIG. 9A not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

FIG. 9B is a graph illustrating a simulated polar profile of luminance output of a transmissive spatial light modulator that is illuminated by a collimated backlight. The display device 100 thus further comprises a backlight 20 arranged to output light, and the spatial light modulator 48 comprises a transmissive spatial light modulator arranged to receive output light from the backlight 20. The backlight 20 has an output luminance profile having a full width half maximum that is at most 40 degrees. The spatial light modulator may alternatively comprise an emissive spatial light modulator and a parallax barrier 700 as described with respect to FIGS. 12A-B, below.

FIG. 9C is a graph illustrating a simulated polar profile of transmission of the second polar control retarder 300B of FIG. 9A and TABLE 1 arranged between the output display polariser and the first additional polariser wherein the electric vector transmission directions of the polarisers are parallel; FIG. 9D is a graph illustrating a simulated polar profile of reflectivity of the second polar control retarder 300B of FIG. 9A and TABLE 1 arranged between the reflective polariser and the second additional polariser 318B wherein the electric vector transmission directions of the polarisers are parallel; FIG. 9E is a graph illustrating a simulated polar profile of the total reflectivity comprising the reflectivity of FIG. 9D and the Fresnel reflectivity from the front surface of the display device;

FIG. 9F is a graph illustrating a simulated polar profile of transmission of the first polar control retarder 300A of FIG. 9A and TABLE 1 arranged between the input display polariser 210 and the first additional polariser 318A wherein the electric vector transmission directions of the polarisers are parallel; and FIG. 9G is a graph illustrating a simulated polar profile of the logarithm of total output luminance of the spatial light modulator and first and second polar control retarders of FIG. 9A and TABLE 1.

FIG. 9H is a graph illustrating a simulated polar profile of the security level, S of the arrangement of FIG. 9A and TABLE 1 for an ambient illuminance measured in lux that is twice the head-on display luminance measured in nits; and FIG. 9I is a graph illustrating a simulated lateral profile for zero degrees elevation of the visual security factor of the arrangement of FIG. 9A and TABLE 1 for an ambient illuminance measured in lux that is twice the head-on display luminance measured in nits. In comparison to the profiles of FIG. 8H and FIG. 8I, the privacy angle is provided at a smaller polar angle and achieves high image security over a significantly wider polar range with high privacy values.

FIG. 9J is a graph illustrating a simulated polar profile of the security level, S of the arrangement of FIG. 9A for an ambient illuminance measured in lux that is twice the head-on display luminance measured in nits and operated in public mode. In comparison to the arrangement using the wide angle emission of FIG. 8B, the collimated backlight with luminance profile of FIG. 9B achieves improved privacy viewing angles, but has narrower polar range over which other observers can see a high contrast image (S<0.1) in privacy mode. Such a display is well suited to display applications that are primarily for single viewers with occasional use by multiple users.

It may be desirable to provide a switchable privacy display with no reflective polariser 302.

FIG. 10A is a schematic diagram illustrating in front perspective view an arrangement polarisers and polar control retarders for the embodiment of FIG. 5A wherein the first and second polar control retarders are anti-parallel. Features of the embodiment of FIG. 10A not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

FIG. 10B is a graph illustrating a simulated polar profile of luminance output of a transmissive spatial light modulator that is illuminated by a collimated backlight; FIG. 10C is a graph illustrating a simulated polar profile of transmission of the first polar control retarder of FIG. 10A and TABLE 1 arranged between the input display polariser and the first additional polariser wherein the electric vector transmission directions of the polarisers are parallel; FIG. 10D is a graph illustrating a simulated polar profile of reflectivity of the first polar control retarder of FIG. 10A and TABLE 1 arranged between the input display polariser and the first additional polariser; FIG. 10E is a graph illustrating a simulated polar profile of the total reflectivity comprising the reflectivity of FIG. 10D and the Fresnel reflectivity from the front surface of the display device; FIG. 10F is a graph illustrating a simulated polar profile of transmission of the second polar control retarder of FIG. 10A and TABLE 1 arranged between the first and second additional polarisers; FIG. 10G is a graph illustrating a simulated polar profile of the logarithm of total output luminance of the spatial light modulator and first and second polar control retarders of FIG. 10A and TABLE 1.

FIG. 10H is a graph illustrating a simulated polar profile of the security level, S of the arrangement of FIG. 10A and TABLE 1 for an ambient illuminance measured in lux that is twice the head-on display luminance measured in nits; and FIG. 10I is a graph illustrating a simulated lateral profile for zero degrees elevation of the visual security factor of the arrangement of FIG. 10A and TABLE 1 for an ambient illuminance measured in lux that is twice the head-on display luminance measured in nits. The lack of reflective component increases the privacy switch-on angle 420 in comparison to the arrangement of FIG. 9I, however the collimated backlight reduces the privacy switch-on angle 420 in comparison to FIG. 8I. Advantageously a display may be provided with lower reflectivity in comparison to arrangements with reflective polariser 302. Such arrangements may be desirable in certain operating environments such as automotive cabins in bright sunlight.

It may be desirable to provide privacy displays that achieve high security level for off-axis viewing in both landscape and portrait modes of operation.

FIG. 11A is a schematic diagram illustrating in front perspective view an arrangement polarisers and polar control retarders for the embodiment of FIG. 4A wherein the first and second polar control retarders are crossed. Features of the embodiment of FIG. 11A not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

One of the surface alignment layers 417B of the at least one second polar control retarder 300B that is arranged to provide homogenous alignment in the adjacent liquid crystal material 421 has a pretilt having a pretilt direction 419B with a component 419Bx in the plane of the layer of liquid crystal material 421 in a second direction, said first and second directions being crossed. In the embodiments of FIG. 11A said first and second directions are perpendicular.

TABLE 2 In-plane Additional Additional passive Alignment alignment LC layer 314 passive retarder retarder 330 Item Layer type Pretilt direction retardance (range) 330 type retardance (range) 300B 419BB Homogeneous  2° 270° 1250 nm 419BA Homeotropic 88°  90° (700 nm~2500 nm) 330B Negative C- −1000 nm plate (−400 nm to −2100 nm) 300A 419AB Homogeneous  2° 180° 1250 nm 419AA Homeotropic 88°  0° (700 nm~2500 nm) 330A Negative C- −1000 nm plate −400 nm to −2100 nm)

FIG. 11B is a graph illustrating a simulated polar profile of luminance output of a transmissive spatial light modulator that is illuminated by a collimated backlight; FIG. 11C is a graph illustrating a simulated polar profile of transmission of the first polar control retarder of FIG. 11A and TABLE 2 arranged between the output display polariser and the first additional polariser wherein the electric vector transmission directions of the polarisers are parallel; FIG. 11D is a graph illustrating a simulated polar profile of reflectivity of the first polar control retarder of FIG. 11A and TABLE 2 arranged between the reflective polariser and the first additional polariser wherein the electric vector transmission directions of the polarisers are parallel; FIG. 11E is a graph illustrating a simulated polar profile of the total reflectivity comprising the reflectivity of FIG. 11D and the Fresnel reflectivity from the front surface of the display device; FIG. 11F is a graph illustrating a simulated polar profile of transmission of the second polar control retarder of FIG. 11A and TABLE 2 arranged between the input display polariser and the second additional polariser wherein the electric vector transmission directions of the polarisers are parallel; and FIG. 11G is a graph illustrating a simulated polar profile of the logarithm of total output luminance of the spatial light modulator and first and second polar control retarders of FIG. 11A and TABLE 2.

FIG. 11H is a graph illustrating a simulated polar profile of the security level, S of the arrangement of FIG. 11A and TABLE 2 for an ambient illuminance measured in lux that is twice the head-on display luminance measured in nits and the display is operated in landscape orientation; and FIG. 11I is a graph illustrating a simulated polar profile of the security level, S of the arrangement of FIG. 11A and TABLE 2 for an ambient illuminance measured in lux that is twice the head-on display luminance measured in nits and the display is operated in portrait orientation.

FIG. 12A is a schematic diagram illustrating in front perspective view an arrangement polarisers and polar control retarders for the embodiment of FIG. 1A wherein the first and second polar control retarders are crossed and the spatial light modulator is provided by the alternative of FIG. 2. Features of the embodiment of FIG. 12A not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

FIG. 12B is a graph illustrating a simulated polar profile of luminance output of an emissive spatial light modulator without the barrier structure of FIG. 2. The spatial light modulator 48 comprises an emissive spatial light modulator arranged to output light, the display polariser is an output display polariser 218 arranged on the output side of the emissive spatial light modulator 48, as illustrated in FIG. 1A. The emissive spatial light modulator comprises an array of pixels 220, 222, 224 arranged in a pixel layer 214, and the display device further comprises a parallax barrier 700 forming an array of apertures 702, wherein the parallax barrier 700 is separated from the pixel layer 214 by a parallax distance d along an axis along a normal to the plane of the pixel layer 214, each pixel 220, 222, 224 being aligned with an aperture 702.

FIG. 12C is a graph illustrating a simulated polar profile of transmission of the barrier structure of FIG. 2 of light from the pixels of the emissive spatial light modulator. An illustrative example is provided in TABLE 3 where the emissive spatial light modulator 48 and the aligned parallax barrier 700 has an output luminance profile having a full width half maximum that is at most 40 degrees.

TABLE 3 Parameter, x-axis direction Illustrative value Pixel 224 pitch 20 microns Pixel 224 emitting width 10 microns Barrier aperture 702 width 10 microns Barrier separation, d 20 microns

FIG. 12D is a graph illustrating a simulated polar profile of transmission of the second polar control retarder of FIG. 12A and TABLE 2 arranged between the first and second additional polarisers wherein the electric vector transmission directions of the polarisers are parallel; FIG. 12E is a graph illustrating a simulated polar profile of reflectivity of the second polar control retarder of FIG. 12A and TABLE 2 arranged between a reflective polariser and the second additional polariser wherein the electric vector transmission directions of the polarisers are parallel; FIG. 12F is a graph illustrating a simulated polar profile of the total reflectivity comprising the reflectivity of FIG. 12E and TABLE 2 and the Fresnel reflectivity from the front surface of the display device; FIG. 12G is a graph illustrating a simulated polar profile of transmission of the first polar control retarder of FIG. 12A and TABLE 2 arranged between the display polariser and the first additional polariser wherein the electric vector transmission directions of the polarisers are parallel; and FIG. 12H is a graph illustrating a simulated polar profile of the logarithm of total output luminance of the spatial light modulator and first and second polar control retarders of FIG. 12A.

FIG. 12I is a graph illustrating a simulated polar profile of the security level, S of the arrangement of FIG. 12A and TABLE 2 for an ambient illuminance measured in lux that is twice the head-on display luminance measured in nits. Advantageously an emissive display may be provided with a polar profile of security level that is appropriate for both landscape and portrait operation.

FIG. 13 is a schematic diagram illustrating in side perspective view a switchable privacy display component 102 for use with a spatial light modulator 48 comprising a first polar control retarder 300A and a first additional polariser 318A, a reflective polariser 302; and a second polar control retarder 300B arranged between the first additional polariser 318A and a second additional polariser 318B. Advantageously a spatial light modulator 48 may be reconfigured in the factory or in the field for use as a switchable privacy display. Features of the embodiment of FIG. 13 not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

FIG. 14 is a key for the alternative stacking arrangements of FIGS. 15A-F. FIGS. 16A-F, FIGS. 17A-C and FIGS. 18A-F.

FIGS. 15A-F are schematic diagrams illustrating in side view alternatives for optical component stackings for a switchable privacy display 100 wherein the first and second polar control retarders 300A, 300B are arranged to receive light from a transmissive spatial light modulator 48 and backlight 20. Features of the embodiment of FIGS. 15A-F not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

FIGS. 16A-F are schematic diagrams illustrating in side view alternatives for optical component stackings for a switchable privacy display 100 wherein one of the first and second polar control retarders 300A, 300B is arranged to receive light from the transmissive spatial light modulator 48 and the spatial light modulator 48 is arranged to receive light from the other of the first and second polar control retarders 300A, 300B and backlight 20; and FIGS. 17A-C are schematic diagrams illustrating in side view alternatives for optical component stackings for a switchable privacy display wherein the spatial light modulator 48 is arranged to receive light from the first and second polar control retarders 300A, 300B and backlight 20. Features of the embodiment of FIGS. 16A-F and FIGS. 17A-C not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

FIGS. 18A-F are schematic diagrams illustrating in side view alternatives for optical component stackings for a switchable privacy display 100 wherein the first and second polar control retarders 300A, 300B are arranged to receive light from an emissive spatial light modulator 48. Features of the embodiment of FIGS. 18A-F not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

In the alternatives of FIGS. 15A-F, FIGS. 16A-F, FIGS. 17A-C and FIGS. 18A-F, various alternatives are illustrated and can more generally be described as follows.

For each polar control retarder 300A, 300B that comprises a liquid crystal retarder 314A, 314B with two homogeneous alignment layers 417A, 417B then the respective passive retarder 330A, 330B or pair of crossed passive retarders 330AA, 330AB or 330BA, 330BB may be arranged to either receive light from the respective liquid crystal retarder 314A, 314B; or the liquid crystal retarder 314A, 314B may be arranged to receive light from the respective passive retarder 330A, 330B or pair of crossed passive retarders 330AA, 330AB or 330BA, 330BB.

For each polar control retarder 300A, 300B that comprises a liquid crystal retarder 314A. 314B with one homogeneous alignment layer and one homeotropic alignment layer 417A, 417B then preferably the homeotropic alignment layer is arranged between the respective layer of liquid crystal material 421A, 421B and the respective passive retarder 330A, 330B. Advantageously the size of the polar region for reduced luminance in privacy mode is increased.

For the alternatives of FIGS. 15A, 15C, 15E, FIGS. 16A, 16C, 16E, and FIGS. 18A, 18C, 18E, one of polar control retarders 300A, 300B may be arranged between reflective polariser 302 and further additional polariser 318B. Advantageously the reflectivity of the display 100 in privacy mode of operation may be increased, and security factor increased.

The alternatives of FIGS. 15A-F, FIGS. 16A-F and FIGS. 17A-C with the transmissive spatial light modulator 48 and backlight 20 omitted or the alternative of FIGS. 18A-F with the emissive spatial light modulator 48 omitted may provide alternative components such as that illustrated in FIG. 13.

As may be used herein, the terms “substantially” and “approximately” provide an industry-accepted tolerance for its corresponding term and/or relativity between items. Such an industry-accepted tolerance ranges from zero percent to ten percent and corresponds to, but is not limited to, component values, angles, et cetera. Such relativity between items ranges between approximately zero percent to ten percent.

While various embodiments in accordance with the principles disclosed herein have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of this disclosure should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with any claims and their equivalents issuing from this disclosure. Furthermore, the above advantages and features are provided in described embodiments, but shall not limit the application of such issued claims to processes and structures accomplishing any or all of the above advantages.

Additionally, the section headings herein are provided for consistency with the suggestions under 37 CFR 1.77 or otherwise to provide organizational cues. These headings shall not limit or characterize the embodiment(s) set out in any claims that may issue from this disclosure. Specifically and by way of example, although the headings refer to a “Technical Field,” the claims should not be limited by the language chosen under this heading to describe the so-called field. Further, a description of a technology in the “Background” is not to be construed as an admission that certain technology is prior art to any embodiment(s) in this disclosure. Neither is the “Summary” to be considered as a characterization of the embodiment(s) set forth in issued claims. Furthermore, any reference in this disclosure to “invention” in the singular should not be used to argue that there is only a single point of novelty in this disclosure. Multiple embodiments may be set forth according to the limitations of the multiple claims issuing from this disclosure, and such claims accordingly define the embodiment(s), and their equivalents, that are protected thereby. In all instances, the scope of such claims shall be considered on their own merits in light of this disclosure, but should not be constrained by the headings set forth herein. 

The invention claimed is:
 1. A display device comprising: a spatial light modulator; a display polariser arranged on a side of the spatial light modulator, the display polariser being a linear polariser; a first additional polariser arranged on the same side of the spatial light modulator as the display polariser, the first additional polariser being a linear polariser; at least one first polar control retarder arranged between the first additional polariser and the display polariser; a second additional polariser, the second additional polariser being a linear polariser; and at least one second polar control retarder, wherein either: the second additional polariser is arranged on the same side of the spatial light modulator as the first additional polariser outside the first additional polariser, and the at least one second polar control retarder is arranged between the first additional polariser and the second additional polariser; or the display device further comprises a backlight arranged to output light, the spatial light modulator comprises a transmissive spatial light modulator arranged to receive output light from the backlight, said display polariser is an input display polariser arranged on the input side of the spatial light modulator, and the display device further comprises an output display polariser arranged on the output side of the spatial light modulator, the second additional polariser is arranged on the output side of the spatial light modulator, and the at least one second polar control retarder is arranged between the second additional polariser and the output display polariser, wherein each of the at least one first polar control retarder and the at least one second polar control retarder comprises a respective switchable liquid crystal retarder comprising a layer of liquid crystal material and two surface alignment layers disposed adjacent to the layer of liquid crystal material and on opposite sides thereof, one of the surface alignment layers being arranged to provide homogenous alignment in the adjacent liquid crystal material and the other of the surface alignment layers being arranged to provide homeotropic alignment in the adjacent liquid crystal material.
 2. A display device according to claim 1, wherein said one of the surface alignment layers of the at least one first polar control retarder that is arranged to provide homogenous alignment in the adjacent liquid crystal material has a pretilt having a pretilt direction with a component in the plane of the layer of liquid crystal material in a first direction, and said one of the surface alignment layers of the at least one second polar control retarder that is arranged to provide homogenous alignment in the adjacent liquid crystal material has a pretilt having a pretilt direction with a component in the plane of the layer of liquid crystal material in a second direction, said first and second directions being crossed.
 3. A display device according to claim 2, wherein said first and second directions are perpendicular.
 4. A display device according to claim 1, wherein said one of the surface alignment layers of the at least one first polar control retarder that is arranged to provide homogenous alignment in the adjacent liquid crystal material has a pretilt having a pretilt direction with a component in the plane of the layer of liquid crystal material in a first direction, and said one of the surface alignment layers of the at least one second polar control retarder that is arranged to provide homogenous alignment in the adjacent liquid crystal material has a pretilt having a pretilt direction with a component in the plane of the layer of liquid crystal material in a second direction, said first and second directions being anti-parallel.
 5. A display device according to claim 1, wherein the display device further comprises a reflective polariser, the reflective polariser being a linear polariser, and either: said display polariser is an output display polariser arranged on the output side of the spatial light modulator, the second additional polariser is arranged on the same side of the spatial light modulator as the first additional polariser outside the first additional polariser, the at least one second polar control retarder is arranged between the first additional polariser and the second additional polariser, and the reflective polariser is arranged between the first additional polariser and the at least one second polar control retarder; or the display device further comprises a backlight arranged to output light, the spatial light modulator comprises a transmissive spatial light modulator arranged to receive output light from the backlight, said display polariser is an input display polariser arranged on the input side of the spatial light modulator, and the display device further comprises an output display polariser arranged on the output side of the spatial light modulator, the second additional polariser is arranged on the output side of the spatial light modulator, and the at least one second polar control retarder is arranged between the second additional polariser and the output display polariser, and the reflective polariser is arranged between the output display polariser and at least one second polar control retarder.
 6. A display device according to claim 1, wherein the switchable liquid crystal retarder of the at least one first polar control retarder has a retardance for light of a wavelength of 550 nm having a first retardance value and the switchable liquid crystal retarder of the at least one second polar control retarder has a retardance for light of a wavelength of 550 nm having a second retardance value, the first retardance value and the second retardance value being the same.
 7. A display device according to claim 1, wherein the switchable liquid crystal retarder of the at least one first polar control retarder and the switchable liquid crystal retarder of the at least one second polar control retarder each have a retardance for light of a wavelength of 550 nm in a range from 700 nm to 2500 nm.
 8. A display device according to claim 1, wherein each of the at least one first polar control retarder and the at least one second polar control retarder further comprises at least one passive compensation retarder.
 9. A display device according to claim 8, wherein each of the at least one passive compensation retarder of the at least one first polar control retarder and the at least one passive compensation retarder of the at least one second polar control retarder is arranged on the same side of the switchable liquid crystal retarder as the surface alignment layers that is arranged to provide homeotropic alignment in the adjacent liquid crystal material.
 10. A display device according to claim 8, wherein the at least one passive compensation retarder of the at least one first polar control retarder and the at least one passive compensation retarder of the at least one second polar control retarder each comprises a passive uniaxial retarder having its optical axis perpendicular to the plane of the retarder.
 11. A display device according to claim 10, wherein the passive uniaxial retarder has a retardance for light of a wavelength of 55nm in a range from −400 nm to −210nm.
 12. A display device according to claim 1, wherein the spatial light modulator comprises an emissive spatial light modulator arranged to output light, the display polariser is an output display polariser arranged on the output side of the emissive spatial light modulator, the second additional polariser is arranged on the output side of the spatial light modulator outside the first additional polariser, and the at least one second polar control retarder is arranged between the first additional polariser and the second additional polariser.
 13. A display device according to claim 12, wherein the emissive spatial light modulator comprises an array of pixels arranged in a pixel layer, and the display device further comprises a parallax barrier forming an array of apertures, wherein the parallax barrier is separated from the pixel layer by a parallax distance along an axis along a normal to the plane of the pixel layer, each pixel being aligned with an aperture.
 14. A display device according to claim 12, wherein the emissive spatial light modulator and parallax barrier has an output luminance profile having a full width half maximum that is at most 40 degrees.
 15. A display device according to claim 1, wherein the display device comprises a backlight arranged to output light, and the spatial light modulator comprises a transmissive spatial light modulator arranged to receive output light from the backlight.
 16. A display device according to claim 15, wherein the backlight has an output luminance profile having a full width half maximum that is at most 40 degrees.
 17. A display device according to claim 15, wherein: said display polariser is an input display polariser arranged on the input side of the spatial light modulator; the first additional polariser is arranged between the backlight and the input display polariser; and the second additional polariser is arranged on the same side of the spatial light modulator as the first additional polariser between the backlight and the first additional polariser, and the at least one second polar control retarder is arranged between the first additional polariser and the second additional polariser. 